Renewable energy sources have typically the problem of being available in a diluted form in time and space; from the sun, it is possible to directly obtain mean electric powers around 20 W per square meter, while wind farms of traditional aeolian towers generate around 4 W per square meter, and with all other sources like biomass, sea forces, stimulated geothermal sources, levels are lower than one watt per square meter. In order to produce meaningful amounts of energy, machines and system for collecting and converting energy from renewable sources must therefore have big or immense sizes. In case of photovoltaic, or thermodynamic sun energy, it is necessary to occupy large extensions of territory with panels or with mirrors, while with the conventional aeolian system it is necessary to have big towers arranged at distances that are as much as possible nearby, and organised in wind farms.
The most important element in evaluating these works, even before the feasibility and financial return evaluations, is the necessary energy for making the systems. The financial return could be positive even in situations of insufficient energy return, due to market deformations linked to government helps. There could be a situation where the intention of producing energy from a renewable source starts such a burdensome implementing process from the energy point of view that the operating years of the system will not be sufficient to return. In order to more deeply specify the question, it is necessary to refer to concepts, typically described in literature, of ERoEI, emergy, exergy: however, it is not object of this document to further explain these facts.